1. Field of the Invention
The present invention relates to a green light-emitting device formed of gallium phosphide (GaP). More particularly, the present invention relates to a GaP green light-emitting diode having improved electrical characteristics and/or improved brightness.
2. Description of the Prior Art
GaP light-emitting diodes that emit green light are today extensively used in various display devices. GaP green light-emitting diodes have made startling progress in recent years, with higher brightness types being developed all the time. Along with the increases in brightness, the range of applications for GaP green light-emitting diodes has become very broad. However, in order to expand the range of applications still further, the development of even higher brightness light-emitting diodes is being sought. Moreover, in order to be able to provide a stable supply of low-priced GaP green light-emitting diodes that meet market demand, there is a need to develop light-emitting diodes having a structure that greatly reduces the proportion of faulty products.
FIG. 2(a) shows a general structure of a conventional GaP green light-emitting diode. In FIG. 2(a), reference numeral 11 denotes an n-type GaP single-crystal substrate, 12 is an n-type GaP layer (n.sub.1 layer) not doped with nitrogen, 13 is an n-type GaP layer (n.sub.2 layer) doped with nitrogen, 14 is a p-type GaP layer, 15 is a p-electrode and 16 is an n-electrode.
In the GaP green light-emitting diode of FIG. 2(a), the n-type GaP single-crystal substrate 11 is of a type generally used, fabricated using the Liquid Encapsulated Czochralski method, doped with an n-type dopant such as S, Te, Si and so forth, and having a donor concentration of about 1 to 20.times.10.sup.17 cm.sup.-3.
The GaP layers 12, 13 and 14 are generally fabricated by the liquid phase epitaxial (LPE) growth method. Silicon is normally added as a dopant to the n-type GaP layer (n.sub.1 layer) 12 not doped with nitrogen, to provide the layer with a donor concentration of around 0.5 to 10.times.10.sup.17 cm.sup.-3. The n.sub.1 layer 12 is provided to alleviate adverse affects of substrate crystal defects on the crystallinity of the nitrogen-doped n-type GaP layer 13.
The nitrogen-doped n-type GaP layer (n.sub.2 layer) 13 forms the emission layer. To increase the brightness of the GaP green light-emitting diode, it is preferable to reduce the donor concentration of the n.sub.2 layer 13, for which reason the donor concentration is usually set at around 1 to 5.times.10.sup.16 cm.sup.-3. Silicon is the n-type dopant most usually used for the n.sub.2 layer 13. Around 2.times.10.sup.18 cm.sup.-3 nitrogen is added to the n.sub.2 layer 13 by supplying ammonia gas during the liquid-phase epitaxial growth process.
Zinc or other such p-type dopant is normally added to the p-type GaP layer 14 to set the acceptor concentration at around 5 to 20.times.10.sup.17 cm.sup.-3. The p-type GaP layer 14 is usually formed after the n.sub.2 layer 13, for which around 2.times.10.sup.18 cm.sup.-3 nitrogen is added. FIG. 2(b) shows representative impurity concentration profiles in a GaP green light-emitting diode having the foregoing structure. The composition of the above type of GaP green light-emitting diode is disclosed by, for example, JP-B Sho 57-54951.
The GaP green light-emitting diode is fabricated by using LPE to epitaxially grow the above GaP layers on the n-type GaP single-crystal substrate, thereby forming an epitaxial wafer, and gold alloys such as AuGe or AuSi, and AuBe or AuZn are then formed on the n-type and p-type sides, respectively, of the epitaxial wafer by vapor deposition, heat treatment and photolithography, to thereby form the p-electrode 15 and n-electrode 16 shown in FIG. 2(a). The wafer is then separated into the individual devices.
In JP-B Sho 57-54951, the reason for the need to reduce the donor concentration in the nitrogen-doped n-type GaP layer of the GaP green light-emitting diode to obtain higher brightness is described as being because there is a reverse correlation between the donor concentration and the nitrogen atomic concentration both in the n-type GaP layer, so that the nitrogen atomic concentration can be raised by lowering the donor concentration. Namely, the nitrogen in the GaP layer functions as an emission center. Also, in the GaP green light-emitting diode, the emission layer is the n.sub.2 layer. Thus, the brightness of the light-emitting diode can be increased by lowering the donor concentration in the n.sub.2 layer to raise the nitrogen concentration.
The present inventors conducted various experiments and studies aimed at developing GaP green light-emitting diodes having a structure that minimizes the proportion of those having defective properties, especially defective electrical characteristics. One electrical characteristic defect in conventional GaP green light-emitting diodes has been the formation of thyristors. That is, for some reason, in a certain proportion of conventional GaP green light-emitting diodes fabricated by the LPE method, a pnpn structure (thyristor) has arisen, causing negative resistance to be exhibited in the electrical characteristics, thereby rendering the diodes defective.
The present inventors first elucidated what causes such thyristors to be produced. This started with an analysis of the device structure that becomes a thyristor. That structure is illustrated by FIG. 3(a). In FIG. 3(a), reference numerals 31, 32, 33, 34, 35 and 36 correspond to the single-crystal substrate 11, n-type GaP layer 12 not doped with nitrogen, n-type GaP layer 13 doped with nitrogen, p-type GaP layer 14, p-electrode 15 and n-electrode 16 of FIG. 2(a). The thyristor in FIG. 3(a) has a p-type GaP inversion layer 37 formed in a portion of the nitrogen-doped n-type GaP layer (n.sub.2 layer) 33 in the vicinity of the interface between the n-type GaP layer (n.sub.1 layer) 32 not doped with nitrogen and the n.sub.2 layer 33. This is the difference between this diode and the normal GaP green light-emitting diode shown in FIG. 2(a). Thus, it is found that devices that become thyristors have a pnpn structure.
To analyze what causes the p-type GaP inversion layer 37 to be formed, secondary ion mass spectrometry (SIMS) was used to investigate the depth profile of impurity concentration in thyristor devices. FIG. 3(b) shows a representative distribution of the impurity concentration in a thyristor device. As shown, in the p-type GaP inversion layer 37 the concentration of carbon, the acceptor impurity, is higher than that of the silicon constituting the donor impurity. The portion of the n.sub.2 layer in the vicinity of the interface between the n.sub.1 layer and the n.sub.2 layer where the carbon concentration is higher than the silicon concentration forms a p-type GaP inversion layer that becomes the pnpn structure.
To improve the brightness of the GaP green light-emitting diodes, the donor concentration of the n.sub.2 layer is normally set at a lower level compared to the n.sub.1 layer. Since the segregation coefficient of the silicon which is the main donor impurity of the n.sub.2 layer has a negative temperature dependency, the concentration of the silicon in the GaP epitaxial layer decreases in portions grown at high temperature and increases in portions grown at low temperature. This means that even in the n.sub.2 layer, the silicon concentration is lower in the vicinity of the interface with the n.sub.1 layer, and higher in the vicinity of the interface with the p-type GaP layer. In GaP green light-emitting diodes the concentration of the donor impurity in the n.sub.2 layer is generally around 1 to 3.times.10.sup.16 cm.sup.-3 in the vicinity of the interface with the n.sub.1 layer, and around 2 to 5.times.10.sup.16 cm.sup.-3 in the vicinity of the interface with the p-type GaP layer. This being the case, when the background level content of the carbon in the GaP layer is around 8 to 20.times.10.sup.16 cm.sup.-3, as in a conventional diode, a p-type inversion layer may be formed within the n.sub.2 layer in the region of the interface with the n.sub.1 layer, producing a thyristor.
A first object of the present invention is to provide a GaP green light-emitting diode having a structure that reduces to very low levels the incidence of electrical characteristic defects caused by thyristors.
The present inventors also carried out experiments and studies aimed at meeting the demand for even higher brightness GaP green light-emitting diodes.
It has heretofore been known that at or above the conventional donor concentration of 1.times.10.sup.16 cm.sup.-3, there is a correlation between the donor concentration in the n.sub.2 emission layer and the diode brightness, and that reducing the donor concentration in the n.sub.2 layer can be used to raise the brightness of the diode. The conventional method used to decrease the donor level in the n-type GaP layer doped with nitrogen was to not add donor impurity to the n.sub.2 layer intentionally.
When donor impurity is not added to the n.sub.2 layer intentionally, the donor concentration in the n.sub.2 layer is mainly determined by (1) the donor impurity eluted from the substrate into the Ga solution used in the epitaxial growth process, and (2) the amount of silicon produced by reduction of the quartz (SiO.sub.2) of the reaction tube in the epitaxial growth furnace with the hydrogen gas and mixed in with the Ga solution. When the n.sub.2 layer is being grown, the silicon included in the Ga solution as a result of the reduction, reacts with the nitrogen to form Si.sub.3 N.sub.4, the major portion being removed by the Ga solution. As a result, the concentration of the silicon taken into the n.sub.2 layer becomes about 1 to 2.times.10.sup.16 cm.sup.-3. When the n-type dopant of the substrate is sulfur, the concentration of sulfur in the n.sub.2 layer as a result of the elution of sulfur from the substrate is about 1 to 3.times.10.sup.16 cm.sup.-3.
The elution of the sulfur from the substrate has been the main reason for not being able to reduce the donor concentration in the n.sub.2 layer. To counter this, JP-A Hei 6-120561 describes providing an n-type GaP buffer layer doped with Te or the like between the substrate and the n.sub.1 layer, and lowering the sulfur concentration in the buffer layer to minimize the amount of sulfur entering the n.sub.2 layer from the substrate. In the GaP green light-emitting diode thus obtained, the main donor impurity in the nitrogen-doped n-type GaP layer is silicon, in a concentration of around 1 to 2.times.10.sup.16 cm.sup.-3. The n.sub.2 layer has a sulfur concentration of around 1.times.10.sup.16 cm.sup.-3.
The present inventors carried out further studies aimed at improving the brightness of GaP green light-emitting diodes. As a result, it was found when the sulfur concentration in the nitrogen-doped n-type GaP layer was reduced in accordance with the method described by JP-A Hei 6-120561, both silicon and sulfur coexisted in the nitrogen-doped n-type GaP layer, but that even when silicon became the main donor impurity, the sulfur in the layer had an effect on the brightness exhibited by the diode. That is, even when the nitrogen-doped n-type GaP layer contained around 1 to 2.times.10.sup.16 cm.sup.-3 silicon and around 1.times.10.sup.16 cm.sup.-3 sulfur, the sulfur concentration altered the brightness of the GaP green light-emitting diode, with lower sulfur resulting in higher brightness.
A second object of the present invention is to provide a GaP green light-emitting diode having improved brightness, by suppressing the entry of sulfur into the nitrogen-doped n-type GaP layer (n.sub.2 layer) to obtain an n.sub.2 layer in which the concentration of sulfur is kept to not more than 6.times.10.sup.15 cm.sup.-3 on the basis of the fact that in the region of the n.sub.2 layer having a sulfur concentration of not more than 1.times.10.sup.16 cm.sup.-3, the sulfur concentration in the n.sub.2 layers and the light-emitting diode brightness have a correlation.